The present invention relates to an air-fuel ratio control system for an internal combustion engine, which controls the air-fuel ratio of air-fuel mixture to a value approximately equal to the stoichiometric air-fuel ratio at which a three-way catalyst acts most effectively.
In a conventional air-fuel ratio control system, the air-fuel ratio of air-fuel mixture burned in cylinders of engine is detected as oxygen concentration of exhaust gases by means of an O.sub.2 -sensor provided in the exhaust passage of the engine at a position upstream of a catalytic converter, and a comparator compares the output signal from the O.sub.2 -sensor with a reference value which is fed from a middle value detecting circuit and produces an output representing whether the signal is greater or smaller than the reference value corresponding to the stoichiometric air-fuel ratio. An electromagnetic value is operated in dependency on the output for regulating the air to be mixed with the mixture to provide the stoichiometric air-fuel ratio.
In such a control system, the output of the O.sub.2 -sensor includes noise and the waveform of the output does not have a steep variation. Therefore, in order to control the air-fuel ratio with a microcomputer, a filter and an A/D converter must be provided between the O.sub.2 -sensor and the microcomputer. Accordingly, the system becomes complicated in construction.